1. Field of the Invention
This invention is in the field of current transformers and, more particularly, phase angle error compensation for current transformers.
2. State of the Prior Art
Current transformers are common devices used for measuring alternating current (AC) flow in electric wires or bus bars. A typical current transformer comprises a magnetic core, a primary winding (which may be the high power wire or bus bar), and a secondary coil wound around one or more sectors or sections of the magnetic core. Essentially, a current transformer outputs a small current that is proportional to a larger current flowing in an electric wire or bus bar, and the use of a burden resistor on the output can provide a low voltage signal that is proportional to the current flowing in the high power electric wire or bus bar. Such small current or low voltage output signals from the current transformer can be used in a variety of instrumentation and control applications, including, for example, measuring and metering the amount of electric current flowing to a load or measuring the amount of power used by a load.
The output signal produced by a current transformer is ideally proportional to and in phase with the primary current. However, in actuality, all current transformers introduce errors which show up in the secondary output signals. Errors commonly specified for a current transformer are the ratio error and the phase error. Ratio in this context refers to the ratio of the primary current in the primary winding, e.g., high power wire or bus bar in which current is being measured, to the output current (or voltage) from the secondary winding. Ideally, the ratio of primary turns (commonly one turn) to secondary turns should define or scale the primary current in the primary winding down to the secondary (output) current. However, due to flux leakage, core losses, magnetizing current, and effects of the burden impedance, the secondary (output) current is less than ideal. For example, a current transformer with one primary turn and 100 secondary turns has a turns ratio of 1:100 and an ideal current ratio of 100:1. Therefore, such a current transformer would ideally output a 1 ampere current signal when the primary winding (e.g., high power wire or bus bar) is carrying 100 amperes of current, or it would ideally output a 5 ampere current signal when the primary winding is carrying 500 amperes of current. However, due to the factors discussed above, it is unlikely that the secondary (output) current signal will be exactly 1 ampere in the first example above or exactly 5 amperes in the second example. The difference between the ideal primary to secondary (output) current ratio and the actual primary to secondary (output) current ratio is the ratio error. Obviously, such ratio errors affect the accuracy of the current measurements made with current transformers, so efforts are usually made to keep such errors to a minimum and/or to compensate for them. It is common to use a ratio-correction factor defined as that factor by which the theoretical ratio of a current transformer must be multiplied to obtain the true ratio. Hence, if the ideal ratio is 100:1 as in the example above, while the actual ratio is 100:0.99, the ratio-correction factor would be 1.01, i.e. 100/0.99=1.01 (rounded). The ratio-correction factor can be applied to the current output measurement to correct the actual measurement to a corrected measurement.
These same parameters that cause the ratio error, e.g., magnetizing current and flux leakage, also cause a phase shift error to occur, which is sometimes called phase angle error or simply phase error. The phase angle error is the phase angle between the AC primary current and the secondary output signal. In a current transformer, a primary current produces a magnetic flux in the transformer core. A changing magnetic flux will induce a voltage across the secondary winding on the core. According to Faraday's Law, this induced voltage will equal the negative of the number of turns in the coil multiplied by the time rate of change of the magnetic flux. If the secondary coil is short-circuited or connected across a resistor, then the induced voltage will generate a current in the secondary coil. Lenz's Law states that the induced voltage generates a current, the magnetic field of which opposes the magnetic field induced by the primary current. However, some of the primary current goes to supplying the magnetic flux to generate the coil voltage (magnetizing flux), and some shows up as core losses due to eddy currents and hysteresis in the core. After the magnetizing flux and core losses are removed, this reduced primary current is the current that produces a magnetic flux in the core, which is opposed by the secondary current. Since the measured primary current does not include the inductive magnetization current, it will generally have a leading phase angle error compared to the actual primary current. Such phase angle error is not a significant factor for accuracy in current measurement, but it can be a substantial factor in power measurements. An uncompensated phase shift can lead to large errors in measurement of real power and power factor. These errors are typically higher in split-core current transformers than in solid core current transformers due to the air gaps in split cores, which reduce the core permeability, reducing the magnetizing inductance, increasing the magnetizing current, thereby increasing the phase angle error.
Accurate measurement and monitoring of electrical energy usage has been important in the past, but it is becoming even more important with expected continued increases in costs and prices of electrical energy. Errors in such measurements, when used as a basis for billing for electrical energy usage, can result in overcharging or undercharging energy users. Consequently, utility companies and other users of electric energy monitoring devices need monitoring devices with highly accurate current transformers that introduce minimal errors into overall monitoring system errors and preferably at minimal extra cost.
There are many methods used to reduce the ratio error and the phase angle error of current transformers. Methods to correct the ratio error can vary depending on the type of output. A current transformer providing a current output can be adjusted by modifying the primary to secondary turns ratio to achieve the desired output current for full scale primary current. A voltage output current transformer can be adjusted by changing the turns ratio and also by scaling the output burden resistor. Also, a ratio-correction factor as described above can be applied to the current output measurement to correct the actual measurement to a corrected measurement.
Phase angle errors in current transformers can be reduced by reducing the magnetizing current, for example, by making the magnetic core larger, by using more copper in the windings, by using higher permeability core materials, or by eliminating or reducing the size of gaps in the core. Of course, larger magnetic cores and more copper in the windings are more expensive and make current transformers physically larger, thus more difficult to fit into often cramped electrical panels. Higher permeability core materials can have much higher costs. Phase angle errors in current transformers can also be minimized or reduced to near zero by using phase compensation. There are many methods in common use for phase compensation in current transformers, including, for example, fairly simple passive phase correction methods using only resistive-capacitive-inductive components in some output circuit topologies, as well as more complex methods employing, for example, active phase compensation where added circuitry supplies the magnetizing current or inserts a phase correction.
Passive phase compensation methods generally add only minimal cost and complexity to current transformer output circuits and are usually preferred over active methods, if they can reduce phase errors sufficiently for the accuracy required for a particular application. One of the simplest passive phase compensation methods places a capacitor across the secondary output, as described by H. W. Price and C. Kent Duff, “Minimizing the errors of current transformers by means of shunts,” University of Toronto Eng. Res. Bull., No. 2, pp. 216-231 (1921) (Library of Congress TK2551, P7). See also, B. Hague, “Instrument Transformers: Their Theory, Characteristics and Testing: A Theoretical and Practical Handbook for Test-Rooms and Research Laboratories,” Chapter III, pp. 78-81, Pitman Publishing Corp., New York (1936). Since the current transformer magnetizing current is inductive, adding capacitive correction can cancel the effect of the inductive magnetizing current as measured at the output.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.